PROJECT SUMMARY One-third of the world's population are infected with the bacterium that causes tuberculosis (TB), and ~10.4 million people develop disease each year and 1.3 million die. In pulmonary TB, the collagenase MMP-1 is the primary effector of lung tissue destruction and cavity formation. Platelets may contribute to the MMP-mediated tissue degrading phenotype seen in TB, as they recruit MMP-secreting leucocytes, upregulate leucocyte MMP secretion, and secrete MMPs themselves, and they may be involved both in inflammation and the control of its resolution. There is a growing interest in developing host-directed therapies that modify the immune response to TB, with the aim of limiting inflammation and enhancing the effectiveness of existing anti-TB drugs. For this, a detailed understanding of TB immunopathology is vital. We propose to investigate platelet activity in TB patients by correlating changes in markers of platelet activation with measures of tissue destruction and clinical/radiological outcomes, focusing on markers identified through our in vitro work and RNASeq. Our overarching hypothesis is that platelets regulate the innate immune response to TB. Hypothesis 1: Platelet activation and activity are increased in TB-infected individuals. Specific Aim 1: To investigate the relationship between platelet activity and tissue destruction in newly diagnosed patients with pulmonary TB compared to healthy controls and to individuals with non-TB respiratory disease (symptomatic controls). Specific Aim 2: To investigate dynamic changes in platelet activity in TB patients during treatment. Hypothesis 2: Specific platelet-leukocyte signalling pathways will be differentially utilised in TB patients compared to healthy controls and to patients with non-TB respiratory disease. Specific Aim 3: Signalling pathways that are identified using RNASeq and/or cellular experiments as important in platelet-leukocyte interactions will be investigated in patients with TB by measuring downstream associated mediators in blood and BALF. Pathways that may be therapeutically interrupted will be identified for further investigation as potential targets for a host directed therapy. We believe that platelets are an important but neglected component of TB immunopathology. We anticipate that we will find significant differences in platelet activity and activation in TB patients compared to patients with non-TB pulmonary disease and healthy controls. We also expect that this will correlate with disease severity at diagnosis and with loss of lung function at completion of treatment, and will improve with treatment. This study will provide novel information and will highlight potential targets for modification via a host- directed therapy, which will be evaluated in an intervention study in the future.